Peter, I've been pouring through my various ID books and have come up with Leccinum pseudoscabrum. Unlike other Leccinums, it's associated with hornbeam, just like the fruiting body you found. Collins guide says its flesh turns blackish when cut, just like yours. http://www.first-nature.com/fungi/leccinum-pseudoscabrum.php

Bruce, well done! I must do some more reading and not just assume something 'common' is the only thing lurking about. I did not think there was much, if any, Birch in Trawscoed wood. Good sassociation with Hornbeam. Kibby says --a very easily recognised species - the brown, glaborous, but often lumpy cap with flesh staining viole-black is distinctive. It is one of the earliest boletes to appear in summer. Just 261 FRDBI records but 6,000 for L.scabrum.

Thank you Philip and well spotted you and Peter. I love it when new species reveal themselves here. Maybe they have been fruiting here before but it's certainly a case of the more you look the more you see.